feat(isPostalCode): Add KR (Korea) postalCode validator and test.

[greatSumini]

I add KR (Korea) postalCode validator and test.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1628 (d2a7285) into master (63b6162) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1628   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          100       100           
  Lines         1843      1843           
=========================================
  Hits          1843      1843           

Impacted Files	Coverage Δ
src/lib/isPostalCode.js	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 63b6162...d2a7285. Read the comment docs.

[tux-tn]

LGTM! Thank you for your contribution and congrats for your first PR here 🎉

[profnandaa]

Thanks for your contrib, and welcome to the project! 🎉